it is essential for a group-buying site to have the tipping-point groupbuying feature:
STORE will create the deal, the deal should include a time-period with starting and ending date, selling price, discounted price, discount percentage,minimum number of sells, max_num of sells, and a fee (as commission paid to the site that displays the deal)
users will purchase, they will receive the coupon code via email. And also the same email will notify that the deal is not yet fulfilled(it will be fulfilled only when the number of minimum purchases is met).
When minimum number_of_sells is met in given time, users will receive mails that the deal is fulfilled. Else the deal is CANCELED. and the my orders section should indicate the deal's status as either pending, fulfilled, or cancelled.
as for how to charge users for their purchases, here are two possible scenarioes:
users will be charged a small amount(exactly as the commission fee) when they make the purchase. If the deal is cancelled, this amount will be either refunded or transferred into credit points so that they can cash later during their next purchases. If the deal is live, users will be charged the exact amount as indicated in the deal, plus the commission fee(which has already been charged).
when users make their purchase, users will be charged for the money as advertaised in the deal,optionally plus the commission fee(or the commission fee can be left to be calculated in the backend in the admin section, between the store and the site owner). if the deal is cancelled, user will get a refund, optionally minus the commission fee.
either way, it involves two-step payment and should be quite complicated, however, it is an essential feature for a group buying site..as for the commission fee, it might not be neccessary if the site chooses not to charge the store by commission per deal..